1. Field of the Invention
The present invention relates to a boost and buck chopper type potential source excitation system for a synchronous type electric power generator, and in particular, to an improved boost and buck chopper type potential source excitation system for a synchronous type electric power generator which is capable of having an operational characteristic which is better than that of a conventional voltage source excitation system with respect to a rated input voltage and forcing an excitation field by boosting an excitation input voltage using a boost chopper when an excitation input voltage is decreased due to a failure in a cable.
2. Description of the Conventional Art
The electric power P which is generated by an electric power generator is determined by a transmission terminal voltage E.sub.s, a receiving terminal voltage E.sub.r, and a value P=(Es.times.Es.times.sin .theta.)/Xe which is obtained by multiplying a since value of a transmission terminal voltage phase difference angle and then dividing the value by a cable impedance X.sub.e. Therefore, as the terminal voltage of the electric power generator which is a transmission terminal voltage, is decreased, the transmission electric power is decreased thereby. In addition, the excitation input voltage and the excitation current are decreased thereby. As the excitation current is more decreased, the terminal voltage is more decreased thereby. The electric power generator is operated at a high speed due to an imbalance between the mechanical input and electrical output of the synchronous electric power generator. If the sing value exceeds an angle of 90.degree., the operation of the electric power generator is synchronously interrupted, and the electric power generator becomes out of control from the system, thus causing an unstable system.
FIG. 1 illustrates a conventional potential source excitation system for a synchronous type electric power generator.
As shown therein, the conventional potential source excitation system for a synchronous type electric power generator includes an electric power generator 2, a buck transformer 5 for buck-transforming an output voltage from the electric power generator 2 into a voltage for exciting the electric power generator 2, a thyristor 10 for supplying the output voltage from the voltage transformer 5 to a field element 2a of the electric power generator 2, an initial excitation switch 1 for supplying an electric power to the field element 2a of the electric power generator 2 for an initial excitation, an ignition angle controller 9 for controlling a gate of the thyristor 10 in order to control an excitation voltage supplied to the field element 2a of the electric power generator 10, a current transformer 3 for detecting an output current from the electric power generator 2, a transformer 4 for detecting an output voltage from the electric power generator 2, an automatic control setting unit 6 for setting a control value for automatically controlling the output voltage from the electric power generator to a predetermined voltage, an automatic controller 7 for receiving a detection current from the current transformer 3, a detection voltage from the transformer 4 and an output voltage from the voltage transformer 5, receiving a control value from the automatic control setting unit 6, and outputting a control signal for controlling an excitation current in order for the output voltage from the electric power generator 2 to be a predetermined voltage, a manual control setting unit 11 for setting a control value in order to manually control the output voltage from the electric power generator 2, a manual controller 12 for comparing the control value from the manual control setting unit 11 with an output voltage from the thyristor 10 and generating a control signal in order to control an ignition angle, and an ignition angle controller 9 for selectively receiving a control signal from the manual controller 12 or a control signal from the automatic controller 7 through a selection switch 8 which is manually operated by an operator and controlling an ignition angle of the thyristor 10.
The initial excitation switch 1 is a switch for supplying an initial driving excitation power to the field element 2a of the electric power generator 2 when the electric power generator 2 is in a non-voltage state. After the initial excitation is performed, when the electric power generator 2 is normally operated, the output voltage from the electric power generator 2 is bucked by the potential power transformer 5, and the excitation power is supplied to the field element 2a of the electric power generator 2 through the thyristor 10. At this time, the gating angle of the thyristor 10 is controlled, and the current of the field element 2a of the electric power generator is controlled. In the normal state, the automatic controller 7 generates an ignition angle control signal of the thyristor 10 so that the output voltage from the electric power generator 2 becomes a set value of the automatic control setting unit 6 which is a request set value, and the ignition angle controller 9 controls an ignition angle of the thyristor 10 in accordance with the gating angle control signal.
In addition, if a failure occurs in the automatic control system, or when an operator wishes to manually control the electric power generator terminal voltage, the switch 8 is operated so that the control signal from the manual controller 12 is inputted into the ignition angle controller 9. The manual controller 12 outputs a control signal to the ignition angle controller 9 in accordance with a control value from the manual control setting unit 11, and the ignition angle controller 9 controls the ignition angle of the thyristor 10.
In the conventional potential source excitation system for a synchronous type electric power generator, an electric power is received from the output terminal of the electric power generator 2 through the buck transformer 5, and the excitation power is supplied to the field element 2a through the thyristor 10. In addition, the automatic controller 7 controls the gate of the thyristor 10 through the ignition angle controller 9 in accordance with the control value from the automatic control setting unit 6 so that the output voltage from the electric power generator 2 becomes a predetermined voltage.
As described above, the conventional voltage potential source excitation system has an advantage in that the construction of the system is simple and it is easy to maintain the system. However, since the excitation power is fed back from the terminal of the electric power generator, the system may be directly influenced by the variation of the terminal voltage of the electric power generator. Therefore, if the terminal voltage is decreased due to a three-phase short circuit, etc. in the output terminal of the electric power generator or in the transmission cable system, the excitation is reinforced in order to maintain a predetermined terminal voltage in the control system. However, in this case, the excitation current becomes shortage due to the decrease of the terminal voltage, namely, the excitation input voltage. Therefore, even though the excitation current becomes shortage, since there is not a mechanical rotation in the electric power generator at a short period, a synchronous failure may occur in the electric power generator due to the energy difference, whereby it is impossible to obtain a stable operation of the electric power generator.
In order to overcome the above-described problems, the current transformer is installed in the terminal of the electric power generator, so that the current source is obtained and is combined with the current voltage source in a vector method, thus supplying an excitation current. In addition, a compound source excitation system is disclosed in order to reinforce the excitation current which is supplied to the field element when a voltage is decreased due to a failure in the cable. However, this method has a disadvantage in that the capacity of the current transformer is increased, and it is impossible to correct a control integer. Therefore, this method is applicably for only a small capacity system.